In recent years, increasing efforts have been made to effect bleaching of pulp with substances free of chlorine. The bleaching agent used is usually ozone, but NO.sub.x, in particular NO.sub.2, is also used. Bleaching with ozone is frequently carried out in the gaseous phase which requires comparatively high consistencies of fibrous materials. It is very important that pulp and ozone be intimately mixed to provide uniform delignification with a reduced amount of chemicals. The pulp fibers must be rendered freely accessible to ozone. This can be achieved, as is known in the art, by using a fluffer, or defibrator, in which the pulp is fluffed, i.e., separated into small bundles of fibers, by means of rotating elements.
Optimum delignification by means of ozone is achieved at room temperature or lower temperatures. This can be achieved in a pulp mill only with increased expenditure because following washing and sorting, the pulp reaches the bleaching station at markedly increased temperatures, especially if the mill is operated in a closed process. A pulp mill normally operates at temperatures between 60.degree. C. and 80.degree. C. and ozone will decompose at temperatures above 40.degree. C. If optimum mixing of fibers and ozone is effected rapidly, the temperature-dependent degradation rate of ozone can be kept very low. Due to the high reactivity of ozone, the bleaching period as compared to other bleaching methods can be kept very short, in the range of seconds to minutes. The admixture of ozone into the pulp fibers and the reaction itself, as already mentioned, should be carried out at the lowest temperatures feasible.
Previously, it has not been possible to mix gases with fibrous materials having high dry solids content during the comminuting operation without the presence of considerable amounts of carrier gas, for instance air, whereby a mixture, containing for example, ozone, oxygen and air, is generated during comminuting or treatment, in particular in a reaction chamber. This is particularly undesirable in a bleaching operation. Moreover, this necessitates a large exhaust gas purification plant.